The invention relates to a process for uniform heating, in particular for the purpose of preserving, of organic products (e.g. foodstuffs which are filled into a container or a plurality of containers which are permeable for alternating electromagnetic fields, the container or containers being introduced, up to slightly below the level of the top surface of the foodstuff in the container, into a liquid having a suitable dielectric constant and having a low loss factor tan .delta. and an adjustable temperature, whereupon the containers and the foodstuff present therein are exposed to an electromagnetic high-frequency field which acts on the foodstuff from opposite sides through the liquid and through the walls of the container or containers, the containers and the foodstuff present therein being irradiated from above by an electromagnetic ultrahigh-frequency field. Such process has been described in U.S. Pat. No. 3,974,355, which requires that the containers are closed at the top, the space above the top level of the goods being filled with a gas, e.g. air.
During uniform heating, in particular for stabilizing or preserving water-containing foodstuffs, problems arise especially if the foodstuff is not completely homogeneous with respect to the dielectric loss factor tan .delta., such as is approximately the case with yoghurt. Further problems arise if the foodstuff has been introduced for the said treatment in packages which are not completely closed.
The above-mentioned prlbems arise in particular in the treatment (whenever "treatment" is mentioned this is always to be understood as a uniform heating, in particular for stabilizing or preserving water-containing foodstuffs) of jams which still contain whole fruit which, during the treatment, are to be preserved more or less completely as whole fruit. The deviations in the tan .delta. values are to be understood, still in the above example, in the sense that in a jam the dielectric loss factor tan .delta. of fruit juice with added sugar differs from that of fruit pieces or entire fruit also present in the jam.
In the conventional heat preservation of foodstuffs by using infrared radiation or contact heat, a deterioration in the quality of the peripheral layers of the goods is almost unavoidable, in the case of packaged foodstuff, as the result of heating to excessive temperatures or for excessive periods. Unpackaged foodstuffs, however, can be brought more or less accurately to the desired average temperature level, and kept there, by constant stirring during the heating process. This advantage is, however, frequently accompanied by the drawback that the mechanically less resistant components of the foodstuff, for example strawberries in a jam, are damaged or even pulped during stirring. The more viscous or pasty the foodstuff, the more this drawback manifests itself.
Since however, the quality of a jam is assessed, e.g., by the number of largely undamaged fruit contained therein, stirring can easily entail a significant reduction in the market value of the jam.
The fruit preserve industry is thus continually searching for a balance between the damage due to overheating and the damage due to stirring. A further problem of this industry is the filling of the treated, sterilized foodstuff into small containers without the risk of reinfection with bacteria or bacterial spores during the filling process if, to save costs, the foodstuff had been prepared and stabilized in containers holding large quantities.